We know that the process by which Ireland was peopled was many layered. This is also the assumption of its origin story, Lebor Gabála Érenn (the Book of Invasions).
Here are my raw and speculative conjectures on how this originally oral tradition of legendary history might be connected to reality:
* The Fomorians may have been the Mesolithic hunter-gatherer people of Ireland. They were by tradition a seafaring people and in some traditions arrived from North Africa both of which would be consistent with what we know of the Atlantic Mesolithic people who left traces such as mtDNA V from North Africa to Arctic Saami people of Finland. There were only a few thousand European hunter-gatherers in Ireland prior to the Neolithic revolution.
By tradition they were the first people in Ireland after Noah's flood before the Partholón and only after the Cessair (discussed below). Western European hunter gathers persisted in Ireland past the fall of the first farmers for about 6,000 years, and would have been racially much more distinctive than later waves of invaders would have been from each other which could account for their physical descriptions as rather grotesque despite the fact that they can produce children with the invaders.
Incidentally, while Ireland was not depopulated by a flood, it was completely depopulated for thousands of years from about 28,000 BCE to 15,000 BCE when it was covered by ice sheets (a frozen flood for those who would like to stretch for a literal correspondence) and then not immediately repopulated when the ice sheets retreated.
* The Partholón may have been the first farmers of Ireland descended from the Cardial Pottery wave of the Neolithic Revolution and arriving via Iberia ca. 4500 BCE whose demise in the story corresponds to the collapse of the first wave farmers of Ireland about ca. 3300 BCE from a peak population ca. 3700 BCE of about 100,000-200,000 people. Their legendary route, by sea from Anatolia to Greece to Sicily to Iberia isn't far from the Cardial Pottery route.
The proprietor of the Old European Culture blog would associate this wave with the first metal age arrivals many centuries later, as they reputedly bring gold to Ireland, but I would note that they do not necessarily more practical metals and that panned gold has a tradition of being retrieved from Caucasian springs since very ancient times and takes less skill to extract than copper or tin. Still, he has a point. Gold was a distinctive feature of Bronze Age Ireland and was rare before then. But, first wave Neolithic people in Ireland, like the Partholón, was a civilization that collapses before subsequent invaders arrived, while Bell Beaker culture was conquered without collapsing in Ireland, and had origins in Anatolia but not in the steppe.
* The Nemed may have been the first Bell Beaker People to arrive in Ireland, by tradition, from the Caspian Sea who clash with the Formorians in multiple battles, and then leave for "the north of the world", for Britain and for Greece. The Greek branch, the Fir Blog return to Ireland after a couple of centuries, and nine of their successive kings rule Ireland for the next 37 years. Their successors, the Tuatha Dé Danann are also descendants of the Nemed, in particular, those who went "into the north of the world" who wrest Ireland from their kin the Fir Bolg and rule another 150 years. In real life, the Bell Beaker people arrive in Ireland around 2500-2200 BCE, but it wouldn't be at all surprising if the continent-wide Bell Beaker civilization, which I've speculated may also have links to the Greek Minoans (like the Fir Blog), comprised factions within themselves who waxed and waned in the Irish political sphere over a period of fifteen centuries or more, about a factor of ten longer than in the story. The Fir Blog are also said in legendary history to be contemporaneous with the Israelite Exodus, which would be about right in this chronology.
* The Milesians, finally, would represent the Celtic speaking Gaels, who arrive in Ireland in reality sometime between the 9th and 4th centuries BCE, quite possibly from Iberia as tradition suggests. This transition, historically, as in the myth, may have been comparatively peaceful reaching a treaty after an early spat of fighting. The association of the Tuatha Dé with the underworld may reference not only their sídhe burial mound practices (in contrast to Indo-European cremation), but also their background as miners of metal from the ground.
This analysis doesn't leave a place for the first wave of invaders, the Cessair, who are descended from Scythians, which would be a likely place of origins for the ancestors of the proto-Celts. This story appears to be derived from the history of the Gaels in the History of the Franks origin story, and the History of the History of Britain's origin story likewise associates this story with the Gaels. Also, notably, these people do not encounter the Fomorians whom later waves of Irish invaders do encounter, and provides origin stories that could plausibly be for all of the people of the world, or at least, all of the Indo-European peoples. This story, therefore, may be out of order and actually represent the prehistory of the Milesians Gaels before their arrival in Ireland, rather than the true first wave of invaders of Ireland.
This analysis gives both populations with genetic steppe origins - the Bell Beakers and the Celts, a steppe origin in legendary history, while denying it to each of the populations which probably didn't have a steppe origin but did expand out of Anatolia as in legendary history.